Add UMA histogram timers for ProcessManager and BackgroundContentsService startup.

extensions/browser/process_manager.cc

Index: extensions/browser/process_manager.cc
diff --git a/extensions/browser/process_manager.cc b/extensions/browser/process_manager.cc
index 5e2e3aa0dcd1397ff692e3fc3ce3c25cdf671ad3..99a82bbdefe3f0547dc2fbf3f315b1a1afa47dc9 100644
--- a/extensions/browser/process_manager.cc
+++ b/extensions/browser/process_manager.cc
@@ -751,7 +751,10 @@ void ProcessManager::Observe(int type,
     case extensions::NOTIFICATION_EXTENSIONS_READY_DEPRECATED: {
       // TODO(jamescook): Convert this to use ExtensionSystem::ready() instead
       // of a notification.
+      const base::TimeTicks start_time = base::TimeTicks::Now();
       MaybeCreateStartupBackgroundHosts();
+      UMA_HISTOGRAM_TIMES("Extensions.ProcessManagerStartupHostsTime",
+                          base::TimeTicks::Now() - start_time);
       break;
     }